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NuSTAR Probes Puzzling Galaxy Merger

This optical image shows the Was 49 system, which consists of a large disk galaxy, Was 49a, merging with a much smaller “dwarf” galaxy Was 49b. Image credit: DCT/NRL

A supermassive black hole inside a tiny galaxy is challenging scientists’ ideas about what happens when two galaxies become one.

Was 49 is the name of a system consisting of a large disk galaxy, referred to as Was 49a, merging with a much smaller “dwarf” galaxy called Was 49b. The dwarf galaxy rotates within the larger galaxy’s disk, about 26,000 light-years from its center. Thanks to NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) mission, scientists have discovered that the dwarf galaxy is so luminous in high-energy X-rays, it must host a supermassive black hole much larger and more powerful than expected.

“This is a completely unique system and runs contrary to what we understand of galaxy mergers,” said Nathan Secrest, lead author of the study and postdoctoral fellow at the U.S. Naval Research Laboratory in Washington.

Data from NuSTAR and the Sloan Digital Sky Survey suggest that the mass of the dwarf galaxy’s black hole is huge, compared to similarly sized galaxies, at more than 2 percent of the galaxy’s own mass.